Skeletal muscle dysfunction is of profound clinical significance in the elderly. The focus of this application is to elucidate mechanisms underlying pathophysiologic adaptation of skeletal muscle to aging and to evaluate the effects of superimposed exercise on these adaptations. Four specific aims are outlined: 1. To elucidate age associated alterations in the expression and activity of the Na-K ATPase. This biochemical evaluation will look at muscle group specificity, subcellular distribution, sub unit association, and pre and post translational mechanisms of modification. 2. To evaluate whether physical activity in skeletal muscle can attenuate or reverse age associated changes in the these various features of Na-K ATPase. 3. To explore whether or not aging is associated with altered potassium balance which would be reflective of altered Na-K pump function. 4. To test the hypothesis that physical activity will improve potassium balance in skeletal muscle during the process of aging. The application will make use of a variety of biochemical techniques and also microdialysis to test tissue potassium fluxes.